Many different lithographic methods have been proposed for producing patterns on a surface. For example, optical lithographic techniques commonly are used in the fabrication of integrated circuits. Optical lithography is the process of transferring patterns of geometric shapes from a mask to a thin layer of radiation-sensitive material (e.g., photoresist) covering a surface (e.g., a semiconductor substrate). In general, optical lithography involves depositing a layer of photoresist on a surface, selectively exposing the photoresist to ultraviolet light through a mask, and selectively removing the exposed (or unexposed) photoresist regions. The resulting patterned resist structure may be processed to transfer (e.g., by etching) the pattern to an underlying layer or to transfer (e.g., by a lift-off process) the pattern to an overlying layer deposited over the patterned resist structure. Current optical exposure tools typically are capable of a resolution of approximately 0.1-1 μm and a registration of approximately 0.5 μm.
Other lithographic techniques have been developed to improve the resolution and registration limitations of optical lithography. For example, x-ray lithography has a resolution of about 0.5 μm (micrometers), or better, and a registration of about 0.5 μm. Ion beam lithography is capable of a resolution on the order of 10 nm (nanometers). Imprint lithography is a non-radiation based lithography technique in which surfaces are modified by contact with a master pattern. The master pattern may lithographic mask the surface directly or it may initiate chemical reactions on a surface. Imprint lithography may be used to create ultra-fine (sub-25 nm) patterns in a thin film.
Referring to FIGS. 1A-1D, U.S. Pat. No. 5,772,905 describes an imprint lithographic process for creating ultra-fine (sub-25 nm) patterns in a thin film 6 that is disposed on a substrate 8. In accordance with this process, a lithographic mask 10 includes a body 12 and a lithographic masking layer 14 that includes a plurality of exposed protruding features 16. In operation, lithographic mask 10 is pressed into thin film 6 to form a relief pattern (FIG. 1B). In one embodiment, features 16 are not pressed all the way into thin film 6 and, consequently, features 16 do not contact substrate 8. After the lithographic mask is removed, thin film 6 has a relief pattern that consists of compressed (or thinned) regions 18 and uncompressed regions 20 (FIG. 1C). Thin film layer 6 may be further processed (e.g., by etching) to expose substrate regions 22 underlying compressed regions 18 of thin film 6 (FIG. 1D). The resulting patterns in thin film 6 may be transferred (e.g., by lift-off processing) to a material that is deposited onto substrate 8. Alternatively, the patterns in thin film 6 may be transferred (e.g., by etching) directly into substrate 8.
As shown in FIG. 2, U.S. Pat. No. 5,772,905 further discloses an alignment system 24 that may be used to align lithographic mask 10 with respect to film 6. Alignment system 24 includes a stationary block 26 supporting substrate 8 and a moveable lithographic masking block 28 carrying lithographic mask 10. A controller 30 controls the operation of an X-Y positioner 32 that is configured to move lithographic masking block 28 in a plane parallel to the supporting surface of stationary block 26, and a Z positioner 34 that is configured to move lithographic masking block 28 in a direction that is orthogonal to the supporting surface of stationary block 26. An alignment mark 36 is carried by lithographic mask 10 and a complementary mark 38 is carried by substrate 18. A sensor 40 on moveable lithographic masking block 28 is coupled to alignment marks 36 and 38. Sensor 40 is configured to provide an alignment signal 42 to controller 30. In one embodiment, sensor 40 is an optical detector and alignment marks 36, 38 are configured to generate a moiré alignment pattern that enables moiré alignment techniques to be used to align lithographic mask 10 with respect to thin film 6. In another embodiment, alignment marks 36, 38 are formed from electrically conducting material, and sensor 40 is configured to detect the capacitance between alignment marks 36, 38. In this embodiment, lithographic mask 10 may be aligned with respect to thin film 6 by moving moveable lithographic masking block 28 until the capacitance between alignment marks 36, 38 is maximized.